In recent years, biodegradable polymer materials have been finding a wide variety of applications including medical materials, drug delivery systems, and environmentally compatible materials. In recent years, in addition to those applications, the biodegradable polymer materials have been requested to provide new functions, and hence various studies have been made.
In particular, the introduction of a chemically modifiable functional group into a molecule of a polyhydroxyalkanoate typified by polylactic acid has been examined. For example, there has been reported a compound into which a carboxyl group or a vinyl group is introduced. For example, polymalic acid has been known as a polyhydroxyalkanoate having a carboxyl group at a side chain thereof. An α-type represented by the chemical formula (14) and a β-type represented by the chemical formula (15) have been known as structures of monomer units for a polymer of polymalic acid.

Of those, a polymer obtained by ring-opening polymerization of a benzyl ester of β-malolactone represented by the chemical formula (16) is disclosed in U.S. Pat. No. 4,265,247 as β-type polymalic acid or a copolymer thereof.
(In the formula R16 represents a benzyl group.)
In addition, a polymer obtained by copolymerization of a six-membered ring diester monomer and a glicolide or lactide as a cyclic diester or a lactone as an intramolecular ring closure reaction ester of ω-hydroxycarboxylic acid represented by the chemical formula (17) is disclosed in Japanese Patent Application Laid-Open No. H02-3415 as a copolymer containing any one of other hydroxyalkanoic acids typified by α-type polymalic acid-glycolic acid copolymer and glycolic acid.
(In the formula, R17 represents a lower alkyl group such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, or a t-butyl group, or a benzyl group.)
“Macromolecules” 2000, vol. 33, No. 13, p. 4619 discloses that 7-oxo-4-oxepancarboxylate is subjected to ring-opening polymerization to produce a polymer having an ester group at a side chain thereof, and the polymer is further subjected to hydrogenolysis to produce a polymer having a carboxylic acid at a side chain thereof as a polyhydroxyalkanoate having a carboxyl group at a side chain thereof. “Biomacromolecules” 2000, vol. 1, p. 275 discloses a polymer in which a benzyloxycarbonyl group is introduced into a methylene group at position α of a carbonyl group in the main chain of poly(ε-caprolactone), the polymer being obtained by: allowing lithium diisopropylamide to react with poly(ε-caprolactone); and allowing the resultant to react with benzyl chloroformate.
“Macromolecular Bioscience” 2004, vol. 4, p. 232 discloses a polymer in which a (benzyloxycarbonyl)methyl group is introduced into a methylene group at position α of a carbonyl group in the main chain of polylactic acid, the polymer being obtained by: allowing lithium diisopropylamide to react with polylactic acid; and allowing the resultant to react with benzyl bromoacetate.
“Polymeric Materials Science & Engineering” 2002, vol. 87, p. 254 discloses, as a polyhydroxyalkanoate having a vinyl group at a side chain thereof, a polymer obtained by ring-opening polymerization of α-allyl(δ-valerolactone).
Similarly, “Polymer Preprints” 2002, vol. 43, No. 2, p. 727 discloses, as a polyhydroxyalkanoate having a vinyl group at a side chain thereof, a polymer obtained by ring-opening polymerization of 3,6-diallyl-1,4-dioxane-2,5-dione as a six-membered ring diester monomer.
There has been reported a polymer having a new function into which a structure providing functionality for a polyhydroxyalkanoate into which a chemically modifiable functional group is introduced as described above is introduced. “International Journal of Biological Macromolecules” 1999, vol. 25, p. 265 discloses the following. A copolymer of α-type malic acid and glycolic acid is obtained by ring-opening polymerization of a cyclic dimer of α-type malic acid and glycolic acid, and the resultant polymer is deprotected to obtain a polyester having a carboxyl group at a side chain thereof. Tripeptide is chemically modified to the carboxyl group at the side chain, and the resultant polymer is evaluated for cell adhesive property. At this time, a good result is obtained.
A large number of electrophotographic methods have been known. A general method involves: utilizing a photoconductive substance to form an electrical latent image on an image-bearing member (photosensitive member) by using various means; developing the latent image with toner to form a visible image; transferring the toner image onto an image-receiving material such as paper as required; and fixing the toner image onto the image-receiving material under heating and/or pressure or the like to provide a copy. Cascade development, magnetic brush development, impression development, or the like has been known as a method of visualizing an electrical latent image. A method involving: using magnetic toner and a rotation developing sleeve having a magnetic pole at its center; and allowing the magnetic toner to fly from a place on the developing sleeve to a place on a photosensitive member in a magnetic field has also been used.
Development methods used for developing an electrostatic latent image are classified into: a two-component development method involving the use of a two-component developer composed of toner and a carrier; and a one-component development method involving the use of a one-component developer composed only of toner and using no carrier. Here, a colored fine particle generally referred to as toner contains a binder resin and a colorant as essential ingredients, and further contains a charge control agent, magnetic powder, or the like as required.
A method of applying charge to toner may involve the utilization of the charging property of a binder resin itself without the use of a charge control agent. However, in this case, stability of charge with time and humidity resistance are poor, and hence it is difficult to obtain a good image. Therefore, a charge control agent is typically added for maintaining charge of toner and for controlling the charge.
Examples of a charge control agent conventionally known in the art include: charge control agents each having negative frictional charging property such as azo dye metal complexes, metal complexes of aromatic dicarboxylic acids, and metal complexes of salicylic acid derivatives; and positive charge control agents such as nigrosin-based dyes, triphenylmethane-based dyes, various quaternary ammonium salts, and organic tin compounds such as dibutyltin oxide. However, toner containing any one of those charge control agents may not sufficiently satisfy quality properties required for the toner such as chargeability and stability with time depending on the composition of the toner.
For example, toner containing an azo dye metal complex known as a negative charge control agent has a certain level of charge amount, but may be poor in dispersibility depending on the kind of a binder resin to be combined because the azo dye metal complex is a low-molecular crystal. In this case, the negative charge control agent is not evenly distributed in the binder resin, and the charge amount distribution of the resultant toner is extremely devoid of sharpness. Accordingly, an image to be obtained has low gradation and is poor in image formation ability. Furthermore, at present, an azo dye metal complex has been used only for toner having a limited hue, which is mainly black, because the azo dye metal complex has an inherent tone. When the azo dye metal complex is used for color toner, there arises a large problem in that the visibility of a colorant necessary for obtaining an image having a tone capable of coping with a severe demand is low.
Examples of a negative charge control agent which is nearly colorless include metal complexes of aromatic dicarboxylic acids. However, dispersibility may be low because a metal complex of an aromatic dicarboxylic acid is not completely colorless and is a low-molecular crystal.
On the other hand, at present, a nigrosin-based dye or a triphenylmethane-based dye known as a positive charge control agent has been used only for toner having a limited hue, which is mainly black, because the dye itself is colored. In addition, the stability of toner with time against continuous copying may not be good. In addition, a conventional quaternary ammonium salt may provide insufficient moisture resistance when it is turned into toner. In this case, stability with time is poor, and repeated use may not provide a good image.
In recent years, a reduction in waste and an increase in safety of waste have been globally perceived as problems from the viewpoint of environmental conservation. Such problems also occur in the field of electrophotography. In other words, as imaging apparatuses have become widespread, waste amounts of printed paper, waste toner that has been already used, and copying paper have increased year by year, and the safety of such waste is an important problem in terms of protection of the global environment.
In view of such a point, polymer-based charge control agents have been examined. Compounds described in U.S. Pat. Nos. 4,480,021, 4,442,189 and 4,925,765 are examples of such charge control agents. Furthermore, a copolymer of: styrene and/or α-methylstyrene; and an alkyl (meth)acrylate or alkyl (meth)acrylate amide having a sulfonic group is generally used as a polymer charge control agent for causing toner to exert negative chargeability. Such a material is advantageous in that it is colorless, but must be added in a large amount in order to obtain a target charge amount.
As described above, each of those compounds does not have sufficient performance as a charge control agent, and is problematic in terms of, for example, charge amount, rise-up property of charge, stability with time, and environmental safety. In addition, when not only a function but also influences on a human body and the environment are taken into consideration, a charge control agent capable of realizing: synthesis using a safer compound; a safer and more moderate synthesis process; a reduction in amount of an organic solvent to be used; and the like has been strongly demanded. However, such a charge control agent and a synthesis process for the charge control agent have not been reported yet, and a function of a charge control agent, contribution of the charge control agent to environmental conservation, and the like are susceptible to further improvement.